This invention pertains to a windshield wiper installation for motor vehicles.
From the German patent specification No. 713 388 a windshield wiper installation is known in which two wipers are each driven by one of two independent motors in pendulum-fashion between a parking position and a reversing position. The motor of the respective leading wiper is switched off when it arrives at a determined angular position and is only switched on again when the other wiper has reached the same position. Thus, the two independent motors can be synchronized for example, in the parking position and in the reversing position. The amount of control for a wiper installation of this kind is relatively small. However, this system can only be used when the wiping areas of these independently driven wipers do not overlap. Otherwise, a collision of the two wipers could occur when the wiping speeds differ considerably.
From the German specification OS No. 26 29 885, a wiper installation is known in which the two motors are controlled by a control device in a way that the two motors remain switched on as long as the wipers move substantially synchronously. However, the quicker motor is switched off until the motor of the slower wiper has reached a corresponding position. For this purpose, pulses released by angular-position-dependent sensors are conducted to two counter stages and the contents of the counters are compared in a comparator. It is, thereby, ensured that the wiping motions of the two wipers are substantially synchronized in the entire wiping area. However, the wipers only move without a jerking action, when a high resolution sensor is used which releases a pulse for the counter at small angular distances. In this case, the wipers are not really brought to a standstill, but only slow down. Such action is hardly noticed by an observer. If, however, a sensor is used which releases only a few pulses in the wiping area, the continuous switching-on and off and the corresponding stopping and starting will be noticed as very undesireable by an observer.
The use of a high-resolution sensor increases the costs of the wiper installation considerably. Opto-electronic transducers must be used because one cannot achieve a sufficient resolution by switching disks of usual size and contact springs sliding thereon.
Sufficient operating reliability is not ensured in the embodiment according to the German specification OS No. 26 29 885. When the supply voltage for the counters is switched off, then switched on again, the counters have a counter reading which does not correspond to the actual angular position. Such a discrepancy can also occur if one wiper is blocked and does not occupy its parking position when the wiper installation is switched off.
Additionally, in the system according to the German specification OS No. 26 29 885, wiper operation is not possible, if one of the two wipers are blocked. In this case, the other wiper will advance to an end position beyond the blocked wiper and then also be switched off.